An oil-impregnated bearing made of a sintered alloy (hereinafter simply referred to as a "sintered oil-impregnated bearing") is a bearing impregnated with a lubricant oil in very numerous open pores existing in the structure of a porous sintered alloy. When using such a sintered oil-impregnated bearing, it is not necessary to supply a lubricant oil from outside, but the lubricant oil impregnated therein permits display of functions as a bearing.
Machines and devices have at present a tendency toward ones lighter in weight and smaller in scale, in addition to the trend toward more complicated mechanisms. Along with this tendency, demand for sintered oil-impregnated bearings is only increasing year by year not only in the light industry producing home appliances and office machines, but also in the heavy industry manufacturing such products as machine tools, civil engineering machines and automobiles, for the purpose of simplifying the lubrication control and saving the required space in machines and devices.
Conventionally, copper-tin sintered alloy (hereinafter referred to as "Cu-Sn sintered alloy") has long been known as a copper-tin type sintered alloy for an oil-impregnated bearing.
In general, when a sintered oil-impregnated bearing is used under severe service conditions or for a long period of time, gradual oxidation and exhaustion of the lubricant oil impregnated therein reduce bearing performance, and finally cause seizure. It is therefore desirable that a sintered oil-impregnated bearing has a highest possible oil content. However, a too high oil content is not desirable because of the decrease in radial crushing strength of the sintered oil-impregnated bearing. For this reason, JIS (abbreviation of the Japanese Industrial Standards) B1581-1976, SBK1218 specifies the lowest allowable values of oil content and radial crushing strength of SBK alloy respectively as 18 vol. % and 15 kg/mm.sup.2, considering the balance between oil content and radial crushing strength. Many of the Cu-Sn type sintered oil-impregnated bearings commercially available at present have an oil content of about 20 vol.% and a radial crushing strength of about 16 kg/mm.sup.2, thus satisfying the oil content and the radial crushing strength specified for SBK alloy. However, when using these conventional Cu-Sn type sintered oil-impregnated bearings under severe service conditions, the insufficient oil content leads to a shorter service life.
Under such circumstances, with a view to reducing the friction at the metal-to-metal contact portion between a rotating shaft and a bearing in the state of boundary lubrication, i.e., in a state in which the rotating shaft and the bearing are partially in direct contact caused by the pressure drop of the lubricant oil film, for extending the service life of the aforementioned conventional Cu-Sn sintered oil-impregnated bearings by improving bearing performance thereof, a Cu-Sn type sintered alloy for an oil-impregnated bearing was proposed, which was added with fine powder of a solid lubricant such as graphite, lead, lead alloy and molybdenum sulfide to the conventional Cu-Sn sintered alloy.
For example, copper-tin-graphite sintered alloy (hereinafter referred to as "Cu-Sn-C sintered alloy") and copper-tin-lead-graphite sintered alloy (hereinafter referred to as "Cu-Sn-Pb-C sintered alloy") are known, and in particular, Cu-Sn-C sintered alloy is most common and widely employed throughout the world. For instance, JIS B1581-1976 sets out a Cu-Sn-C sintered alloy consisting of from 8 to 11 wt.% tin, up to 2 wt.% graphite, up to 1 wt.% iron, up to 0.5 wt.% miscellaneous elements and the balance copper as alloy No. SBK 1218 (hereinafter referred to as "SBK alloy").
The oil-impregnated bearing made of the conventional Cu-Sn type sintered alloy containing a solid lubricant shows an excellent bearing performance within the region of ordinary service conditions, i.e., within the region of service conditions under the state of boundary lubrication including a load, p, of up to 20 kg/cm.sup.2, a peripheral velocity, V, of up to 200 m/minute, and a PV value (product of P and V) of up to 1,000 kg/cm.sup.2. m/minute, whereas in a lower-load and higher-velocity region as compared with ordinary service conditions, i.e., under service conditions including a load, P, of up to about 5 kg/cm.sup.2 and a peripheral velocity of at least about 400 m/minute, the oil-impregnated bearing made of the conventional Cu-Sn type sintered alloy was almost unserviceable as a bearing. More particularly, the above-mentioned sintered oil-impregnated bearing seriously wears in the state of boundary lubrication before transfer to the high-velocity region, i.e., the very severe state of service conditions in which the rotating shaft and the bearing partially come into direct contact without having an intermediary lubrication oil film in between, and is defective because of the bearing performance rather reduced by causes as described later in the state of hydrodynamic lubrication after complete transfer to the high-velocity region, i.e., in the state in which the impregnated lubrication oil forms an oil film having a hydrodynamically sufficient thickness and there is no partial metal-to-metal contact portion between the rotating shaft and the bearing.
When a large quantity of solid lubricant is added to the conventional Cu-Sn type sintered alloy in an attempt to alleviate this severe state of boundary lubrication, the alloy matrix is broken into pieces, resulting in a lower strength of the alloy matrix and also a decreased wear resistance of the bearing. On the other hand, if the forming pressure is increased when forming the green compact with a view to compensating the decrease in the strength of the alloy matrix, the oil content which is indispensable for a sintered oil-impregnated bearing decreases. If, furthermore, the sintering temperature is raised for compensating the decrease in the strength of the alloy matrix, a solid lubricant such as molybdenum disulfide, if used, is decomposed and scatters. Moreover, if lead and/or lead alloy is employed as a lubricating constituent, a higher temperature activates the liquidus sintering phenomenon, thus not only increasing the density of the sintered alloy, but also causing pore closing phenomenon in open pores for impregnating the lubrication oil, hence resulting in a decreased oil content.
In order to improve the strength of Cu-Sn-C alloy, the known practice is to add phosphorus. For example, Japanese Patent Publication No. 451/60 dated Jan. 26, 1960 discloses a sliding plate for collector for an electric car, made of a sintered alloy consisting of from 0.1 to 5 wt.% phosphorus, from 5 to 18 wt.% tin, from 2 to 10 wt.% graphite, and the balance copper. However, the sintered alloy disclosed in said Japanese Patent Publication was developed for a collector sliding plate for an electric car, not for an oil-impregnated bearing. The addition of phosphorus is therefore only to improve the strength and the hardness of a sliding plate. The sintered alloy consisting of 1 wt.% phosphorus, 15 wt.% tin, 5 wt.% graphite and 79 wt. % copper, which is described as the only example in said Japanese Patent Publication, has an oil content of only 1 vol.%. The sintered alloy disclosed in said Japanese Patent Publication is not therefore suitable at all as an alloy for an oil-impregnated bearing.